The present invention relates to insulated coils for electro-dynamic machines. The improved coils are particularly useful as field coils of large direct current machines, such as the traction motors of diesel-electric locomotives, which in operation are exposed to a very hostile environment.
A locomotive traction motor has to be able to provide long periods of full load, trouble free operation under very harsh conditions, including constant vibration, frequent mechanical shocks, infrequent maintenance, occasional electrical overloads, a wide range of ambient temperatures, and an atmosphere that can be very wet and/or dirty. Such a motor typically includes a plurality of field coils respectively mounted on pole pieces that extend radially inwardly from the magnet frame or stator of the motor. Each field coil comprises a long copper conductor of relatively large cross-sectional area (e.g., one-half square inch) that has been wound or bent to form a plurality of juxtaposed turns (e.g. 15 turns). Typically the copper conductor has a rectangular cross section and is wound edgewise, i.e., the wider side of the conductor is perpendicular to the centerline of the coil. The resulting multi-turn coil is helical in shape, and its open center or "window" is appropriately dimensioned to fit around the associated pole piece. The terminal sections of the copper conductor at opposite ends of the coil serve as relatively flexible leads that are adapted to be connected in circuit with a source of unidirectional electric current, and when energized or excited by such current the coil generates a desired magnetic field inside the motor.
Between adjacent turns of the field coil suitable insulating material is disposed so as to prevent turn-to-turn electrical short circuits, and the exposed surfaces of the whole stack of helical turns are encapsulated in such material to insulate the coil from the grounded pole piece and frame of the motor. The insulating system preferrably is characterized by high dielectric strength, good heat transfer properties, both physical and chemical stability at elevated temperatures, and a high resistance to moisture and dirt. The heat transfer properties are particularly significant in traction motors where the goal is to obtain more output torque per unit of weight by increasing the current density (and hence the heat generated) in the coils.
There are several known methods of insulating field coils. A method commonly used to apply turn insulation comprises dipping the bare copper coil in a suitable shellac, placing pieces of insulating paper between adjacent turns, heating the coil so that the shellac melts into the paper, and clamping the turns of the hot coil together while the material between the turns solidifies and forms a solid bond therewith. Subsequently, ground insulation can be applied by wrapping glass cloth or mica or a combination thereof around the outside of the whole stack of turns, vacuum-pressure impregnating the wrapped coil with a polyester resin varnish, and baking the coil to cure the varnish to a solid condition. The latter two steps are usually repeated at least once so as effectively to seal the coil and prevent penetration of contaminants such as dirt and moisture. While it is technically sound and has proven successful, this prior art method requires a relatively large number of steps, uses an undesirable amount of energy, and results in the build up of a thick insulation layer that undesirably impedes the dissipation of heat generated in the coil.
Alternative methods have heretofore been used to apply the ground insulation. In one, mica tape or the like is wound around the coil in successive layers with a protective covering of glass tape overlaying the mica tape. Afterwards, the coil is assembled on its pole piece, the pole piece is bolted in the motor frame, and the completed assembly is vacuum pressure impregnated with varnish to improve moisture resistance and to aid heat transfer. In another known method, the coil turns are completely encased in a composite insulation of glass cloth, mica mat, and epoxy resin which are molded under heat and pressure to form the ground insulation. The finished coil is hard and inflexible, requiring spring pads between the pole tips and the coil to maintain a tight assembly in the motor.
An insulating material that is particularly advantageous for traction motor applications is known generically as polyimide film, and it is manufactured and sold by the DuPont Company under the trademark "Kapton." Thin gauge Kapton insulation has a relatively high dielectric strength and remains stable at elevated temperatures. It can be coated with Teflon.RTM.FEP-fluorocarbon resin to provide a heat-sealable surface on the base of polyimide film. This also improves the chemical resistance of the film and reduces the rate of moisture permeability and of oxidative decomposition. Such composite material has been heretofore used successfully to insulate rectangular motor magnet wire and to insulate the turns of large current smoothing reactors.